Redox flow batteries (RFBs) find application in various operational environments in which high-capacity energy storage is required. Conventional RFBs generally employ ion permeable membranes in order to separate anolyte and catholyte active redox species while retaining sufficient ionic conductivity to allow charge-compensating ions to permeate across the membrane. However, in addition to allowing transport of charge-compensating ions, these conventional membranes are in many cases permeable to the anolyte and catholyte active redox species and/or the solvents in which these species are dissolved. Over time, this crossover mixing of the catholyte and anolyte active species and their solvents degrades the RFB's performance and energy capacity. Furthermore, certain combinations of anolyte/catholyte active redox species or anolyte/catholyte solvents are impractical or impossible to achieve, as these materials may exhibit undesirable or hazardous chemical reactions with one another as a result of crossover mixing.
Crossover of redox active species and solvents can be partially alleviated by using solid state ionic conducting materials such as Lithium super-ionic-conductor (LiSICON) and β″-alumina to separate the cathode and anode portions of the RFB. However, these materials are poorly suited to use in RFBs. LiSICON exhibits relatively low ionic conductivity at room temperature (e.g., on the order of 10−6 S/cm). β″-alumina has low room temperature conductivity, and degrades quickly in the presence of water, thereby necessitating the use of organic solvents that are poor conductors, are expensive, and raise various safety concerns.